It was towed because it was parked (in a legal space) for longer than the condo association says is "allowed" (by their rules), but the key point is that they DO NOT OWN THE PROPERTY on which it was parked. It's a separate tax parcel, on which they are not listed as owners and on which they do not pay taxes and which is not included in the legal description of the condominium.

As far as I can see, what they did is no different than if I were to put up a sign on land I do not own, with an arbitrary time limit for parking there, and then start towing anyone who "violated" my rule. If I were to behave like that, I would rightfullyy be arrested for theft of a vehicle, since I have no right to impose my own set of rules on other peoples' property. Right? What am I missing here?

More importantly: How do I defend my rights against these arrogant, officious fools?

Should I send them some sort of a "Cease and Desist Letter"?

Right now I think they might be able to claim ignorance of the boundaries of the property with some minimal degree of plausibility (though it seems astonishingly incompetent to me to start towing people from the street before first determining with certainty that you actually have the legal authority to do so.) Regardless, to prevent them from doing this again, and proetect my rights in case they do, what sort of letter (and what sort of proof of delivery, receipt, etc.) should I use to establish the fact that regardless of how ignorant they may have been at first, from this moment forward, they have been publicly notified that they have no right to do what they have been doing and that if they persist they will be knowingly violating the law?